1. Field of the Invention
Many instruments and machines are susceptible to damage by shocks and vibrations which occur only when the instrument or machine is in the inoperable condition. Protection against such shocks and vibrations fall into two categories: first, isolation of the sensitive apparatus from the environmental shock or vibration; and second, changing the internal apparatus arrangement in some way to render it less sensitive to shock and vibration which is actually applied to the apparatus frame. This invention relates to the latter solution.
During launch of a rocket or a rocket-boosted vehicle, shock and vibration levels far higher than 1 g are typically experienced. Isolation for these levels is bulky and heavy, and in some cases not permitted because of alignment requirements, or impossible because of the high acoustic noise levels in the vehicle. For precision instruments which have small clearances or precise surfaces, any relative motion between temporarily touching surfaces can cause severe damage or reduction in operating life or performance. This problem can be particularly troublesome for machines such as a Stirling cycle refrigerating machine which may be used to supercool radiation detectors. Such machines may use advanced piston magnetic suspension techniques, and the performance of these can be impaired by even very small rubbing contacts.
A markedly different situation exists with respect to home or office electronic and entertainment machines. Often these utilize "shipping screws" to immobilize vulnerable moving assemblies. While this expedient can be quite effective and inexpensive, marketing of the apparatus may be greatly aided if set-up can be simplified for the customer.
2. Description of the Prior Art
Stirling cycle coolers, having magnetically suspended pistons or displacers, may use the suspension magnets in a "lock-up" mode, to attempt to hold the moving parts tightly against cylinder walls. However, even apparently microscopic relative movements have been discovered to generate microscopic wear particles, which may clog or reduce the effectiveness of heat exchanger surfaces or lead to additional wear. Because these coolers are hermetically sealed, use of ordinary mechanical locks which are situated substantially outside of the machine itself is impossible.
One common expedient for protecting a sensitive instrumentation against damage when not in the operating condition is found in the field of weighing. Most scales utilize one or more levers which enable a relatively small weight at the end of a long lever arm to balance a heavy weight on a short lever arm. The lever arms themselves are defined by distances from what is typically a "knife edge" pivot. These knife edges are readily damaged by shock loads. Therefore, it is common to provide a means of lifting the lever or balance beam off the knife edge when the scale is not in use. Such devices do not, however, provide much help in solving the problem of the thermodynamic machine such as a Stirling cooler which uses a normally non-contacting bearing, and is designed to be protected from any contact between moving parts.
In the entertainment electronics and office machine field, subassemblies which are hung from the machine frame through a vibration isolator, or large movable subassemblies are often blocked against motion during shipment. In most instances, removable shipping blocks or screws are utilized to prevent relative motion during handling and shipment, and these blocks or screws must then be removed by the user prior to initial operation. However, these techniques usually do not result in a degree of relative-motion protection which would suffice under the severe conditions of a rocket launch. The heavy shock and vibration levels encountered under launch conditions may cause parts, which have apparently been temporarily tightly clamped together, nonetheless to undergo a slight relative movement which results in a microscopic scraping movement between the surfaces which are in contact. Any such effect which occurs with respect to an office machine or a home entertainment apparatus is not likely to produce wear particles that will cause performance or life degradation. However, a satellite cryogenic cooler may have operating clearances of the order of 25 microns (1 milli-inch) between non-contacting surfaces bounding a space which communicates with ultralow temperature heat exchanger surfaces. The slightest debris generation in such a device is apt to reduce the low temperature performance.